Spark plug tester



May 15, 1962 J. A. WHALEY ETAL 3,035,224

SPARK PLUG TESTER Filed Nov. 27, 1956 Z/ i? i 1% ff {i W iffy /a" j? 33AW g 52' I! I 7 I g 7 IN VEN TORS A TTORNEY United States Patent Ofihce3,035,224 Patented May 15, 1962 3,035,224 SPARK PLUG TESTER John A.Whaley and Willard R. Houser, Flint, Mich., assignors to General MotorsCorporation, Detroit, Mich., a corporation of Delaware Filed Nov. 27,1956, Ser. No. 624,558 1 Claim. (Cl. 32416) This invention relates toapparatus for ascertaining conditions of a spark plug in a combustionchamber and more particularly for testing spark plugs of internalcombustion engines for pre-ignition, spark plug fouling, and the like.

In studies of the operation of internal combustion engines, forservicing or evaluation, the duration or time period of the combustionin the operating cycle is a characteristic of particular conditions inthe combustion chamber. For example, in operation under conditions ofnormal ignition combustion occurs late in the compression stroke asdetermined by the timing of the spark ignition. Under abnormal ignitionconditions such as pre-ignition, combustion is caused to occur earlierin the compression stroke and the duration of combustion iscorrespondingly increased. 'lhere are, of course, other abnormalconditions in the combustion chamber, such as spark plug fouling, whichare induced by various causes and result in disturbance or change of thecombustion duration.

It has been found that the electrical resistance between spaced pointsin a combustion chamber is related, not only to the condition of thespark plug but also, to conditions attendant with combustion which causea marked decrease of resistance. In accordance with this inventionameasure of combustion duration is obtained by sens ing' the timeaverage value of electrical resistance between spaced points in thecombustion chamber. Thus, a change of ignition conditions may bedetected by the change of resistance. This is accomplished during engineoperation by a meter circuit connected across spaced electrodes,suitably the spark plug electrodes, which develop an indication of thetime average value of resistance therebetween. The meter circuit,including a source of voltage, preferably comprises means for blockingthe ignition voltage from the meter circuit.

A more complete understanding of the invention may be had from thedetailed description which follows taken with the accompanying drawingin which the single FIG- URE is a schematic diagram of the inventivesystem.

Referring now to the drawing, there is shown an illustrative embodimentof the invention in a pre-ignition meter for a spark ignited internalcombustion engine. The system, in general, comprises an engine includinga cylinder 11 provided with a conventional spark plug 12 to which isconnected the usual ignition circuit 14 and the meter circuit 16.

The engine cylinder defines a combustion chamber 18 into which extendsthe spark plug 12 for developing the igniting spark in the usual manner.The spark plug 12 has an electrode 20 connected through the spark plugshell to ground and a center electrode 22 insulated from the shell andextending to the terminal 23. The ignition circuit 14 is suitably ofconventional design and includes the vehicle battery 24 having oneterminal connected to ground and the other terminal extending throughthe igni tion switch 26 to the ignition coil or transformer 30. Theigmtion circuit extends through the primary winding 28 of the coil 30 tothe movable contact of the periodic switch or breaker points 32, theother contact of which is connected to ground. The breaker points 32 areopened and closed periodically by the cam 34 which is drivensynchronously with the engine. A condenser 36 is connected across andaflords protection for the breaker points 32 in the usual manner. Thesecondary winding 38 of the ignition coil 30 has one terminal connectedto a terminal of the primary winding and its other terminal connected tothe rotor contact 40 of a distributor 42. The fixed contact 44 of thedistributor corresponding to the cylinder 11 is connected to theterminal 23 of spark plug 12. The distributor rotor contact 40 is drivenin timed relation with the cam 34.

The meter circuit 16 is connected across the spark plug electrodes 20and 22 and comprises in series connection a voltage source or battery46, a meter 48, a blocking diode 50 and a current limiting resistor 60.The meter 48 is suitably a conventional moving coil type micro-ammeter.The diode 50 is suitably of the high vacuum type and includes a plateelectrode 58 and a thermally emissive cathode 52 provided with a heatercircuit including a voltage source 54 and a switch 56. The battery 46has one terminal connected to ground and its other terminal connectedthrough the meter 48 to the cathode 52 of the diode and the plate of thediode is connected through the resistor 60 to the terminal 23 of thespark plug. It is to be noted that the orientation of the diode 50 inthe meter circuit is such that it will conduct meter circuit currentfrom the battery 46 but will block any current flow in the meter circuitdue to the ignition voltage. For this purpose the polarities of thediode 50 and the battery 46 are of the same sense and are in a senseopposite that of the ignition voltage appearing across the spark plugelec trodes 20 and 22. In the illustrative example, the battery 24 hasits negative terminal connected to ground and the ignition coil 30 isarranged to develop an ignition volt- I age which is negative withrespect to ground. Accor ingly, the diode 50 is oriented with its platereceiving the negative ignition voltage and the battery 46 has itspositive terminal connected to ground to apply a negative voltage to thecathode of the diode.

In operation of the inventive system, with the ignition switch 26 closedand the engine running, the ignition Volt-- age is applied to the sparkplug 12 once each cycle late in the compression stroke of the piston incylinder 10. This is accomplished in the usual manner by the alternateclosing and opening of the breaker points 32 which energizes anddc-energizes the primary winding 28 to cause a high voltage to beinduced in the secondary Winding 36 and applied through the distributor42 to the terminal 23 of the spark plug 12 by the synchronized rotationof the cam 34 and the distributor rotor 40 by the engine. This ignitionvoltage, of negative polarity, is also applied through the resistor 60to the plate 58 of the blocking diode 50. Due to the relative polarityof the ignition volt-age and the diode 50, no current is caused to howin the meter circuit 16.

When the engine is operating under normal ignition conditions, the metercircuit 16 is conditioned for operation by closing the switch 56 in theheater circuit to render the cathode '52 emissive. Accordingly, thebattery 46 will cause a current to flow in the meter circuit from thepositive terminal of the battery through ground to the electrode 20 andthence across the spark plug gap to the electrode 22. The circuit iscompleted from the spark plug terminal 23 through the resistor 60, thediode 50, and the meter 48 to the negative terminal of the battery. Thespark plug gap between electrodes 20 and 22, in air or a gaseousdielectric at low temperature, presents a very high electricalresistance. However when combustion occurs, accompanied by a flame frontand very high temper-atures', the resistance of the gap is substantiallyreduced. Accordingly, an appreciable amount of current may flow in themeter circuit under the influence of the battery 46. Under normalconditions of ignition; that is, when combustion does not precede theoccurrence of the ignition voltage, the meter circuit current has avalue dependent upon the magnitude of the voltage in the meter circuitand the resistance presented by the spark plug gap. As a typical value,the voltage of battery 46 may be 270 volts. The meter 48 responds to atime average value of the current flow in the meter circuit and averagesthe high value of current during combustion with the low value ofcurrent during the remainder of the cycle. A typical range of valueswhich have been found to correspond to normal ignition conditions is ofthe order of to 25 micro-amperes.

When the combustion conditions change in the cylinder, as by theoccurrence of pre-ignition, the flame front and high temperature occursearlier in the compression stroke. Accordingly, the resistance of thespark plug gap assumes a low value for a longer interval during eachoperating cycle. Thus the period of high current fiow in the metercircuit is increased and the time average value, as indicated by themeter 48, is abruptly increased. A typical range of current values inthe meter circuit corresponding to pre-ignition conditions is of theorder of 50 to 100 micro-amperes. Thus, the occurrence of p-re-ignitionor the increased duration of combustion during the engine cycle may bereadily detected by the abrupt change of the average value of currentflow in the meter circuit.

Although the description of this invention has been given with respectto a particular embodiment, it is not .to be construed in a limitingsense. Many variations and modifications within the spirit and scope ofthe invention will now occur to those skilled in the art. For adefinition of the invention reference is made to the appended claim.

We claim:

-Apparatus for detecting preignition and like conditions in operation ofa spark ignited combustion engine comprising, an internal combustionengine with a spark plug having a pair of electrodes disposed in acombustion chamber, an engine ignition circuit connected across theelectrodes of the spark plug to apply a sparking voltage of a givenpolarity across the electrodes, a measuring circuit comprising a firstdirect voltage source, an average current indicating device, a currentlimiting resistor and a unidirectional conductive device connected inseries across the electrodes of the spark plug, the unidirectionalconductive device being connected with a polarity opposite the sparkingvoltage to block current flow from the ignition system through thecurrent indicating device, the unidirectional conductive device beingconnected with the same polarity as said first direct voltage sourcewhereby said first direct voltage source causes a low value of currentflow between the spark plug electrodes when the resistance therebetweenis of high value prior to combustion in the chamber and causes a highvalue of current flow between the spark plug electrodes duringcombustion in the chamber when the resistance therebetween is of lowvalue so that the time average value of the currents, as indicated bythe current indicating device, increases when the resistance between theelectrodes is decreased, by preignition or like conditions, prior to theoccurrence of the sparking voltage from the ignition circuit.

References Cited in the file of this patent UNITED STATES PATENTS2,254,080 McCarty Aug. 26, 1941 2,254,175 Eltgroth Aug. 26, 19412,324,458 Peters July 13, 1943 2,425,321 Horton Aug. 12, 1947 2,543,141Vichnievsky Feb. 27, 1951 2,648,818 Cohen Aug. 11, 1953 2,695,987McCollom et al. Nov. 30, 1954 2,762,977 Krueger Sept. 11, 1956 2,774,063Penn Dec. 11, 1956 2,820,945 Marsden Jan. 21, 1958 2,888,822 BurhansJune 2, 1959 2,908,859 Moehring Oct. 13, 1959 OTHER REFERENCES Article:Note Techniques No. 27, R. Vichnievsky; Methodes de Determination deLintensite de la Detonation; G.R.A.; Paris 3, Rue Leon-Bonnat; 1946.

